The present invention relates to a method for avoiding malfunctions in the solidification of aqueous, radioactive wastes in a glass, glass ceramic or glass ceramic-like matrix in which the wastes are spray-dried and calcinated.
In known processes for the solidification of aqueous, highly radioactive wastes in a glass matrix or in a glass ceramic matrix or a glass ceramic-like matrix, the waste waters generally are first denitrated and the denitrated waste waters, for example, are spray-dried and calcinated in a spray calcinator which contains a spray nozzle. Waste gases are formed during the spray drying and calcination process and are sent to a waste gas filtering system comprised of filter candles or cartridges. The calcinate resulting from this spray drying and calcination process can then be mixed in solid form with glass former substances or with a ground, previously-produced glass frit of a certain composition and can then be melted in a crucible or furnace to form a homogeneous mass. Alternatively, the glass formers or glass frits, respectively, are added to the waste water before the spray drying process. In this case, in order to obtain a good and uniform mixture, of the components being spray dried and calcinated, until the calcinate enter the melting crucible, the additive Aerosil has been added to the waste solution before the spray drying. Aerosil is a particularly pure and finely-dispersed silicic acid which forms colloidal suspensions in aqueous solutions. This method is reported by J. Saidl in an article entitled "Verfestigung hochaktiver Spaltprodukte in Glas" (in translation, Solidification of Highly Active Fission Products in Glass), which appears at pages 33 to 38 of "Jahresbericht 1973, Abteilung Dekontaminationsbetriebe; Bericht der Gesellschaft fur Kernforschung mbH" (in translation, Annual Report for 1973 - Department of Decontamination Operations; Report of the Gesellschaft fur Kernforschung mbH), No. KFK-2126 (May, 1975).
It has now been found, however, that with such a mode of operation, namely, first calcinating and then mixing the calcinate with the glass forming substances or with the previously-produced glass frit, the spray nozzle in the spray calcinator becomes clogged as the time of operation progresses and/or the filter units in the waste gas filtering system grow shut and unavoidable deposits occur in increasing quantities in the calcinator. In the alternative case, when the Aerosil is stirred into the waste solution, its poor miscibility produces difficulties with regard to the fact that, with greater throughputs of waste solution to the spray calcinator, the required good mixing can be attained only after longer periods of dwell. Moreover, in spite of the fact that Aerosil is composed of superfine grains, demixing of the Aerosil can be noted during transporting of the metering quantity of the aqueous waste feeder solution toward the spray nozzle which may again lead to clogging.